The present invention relates to a dental ceramic material for the preparation and repair of metal ceramic and fully ceramic dentures with a processing temperature below 700.degree. C. and a coefficient of thermal expansion .alpha. of 13-14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C.
Using dental ceramic materials, it is possible to prepare fully ceramic dentures, to carry out modifications to change the shape of metal ceramic and fully ceramic dentures, to repair damaged sections or to line metallic dentures with a ceramic layer.
For many years ceramic layers have been used in dentistry for lining metallic frameworks (crowns, bridges), using a type of enamelling, in order to give the denture a natural appearance. In this case, ceramic powders are applied to the metallic framework as an aqueous slurry and fired at elevated temperatures. It is important here that the firing temperature (processing temperature) of the ceramic body is at least 100.degree. C. below the solidus temperature of the material in the metallic framework and that the coefficient of thermal expansion of the ceramic body in the range 25.degree. to 500.degree. C. is only very slightly less than that of the metallic material, so that no cracks are produced in the lining layer during firing and cooling down.
In the same way as these so-called metal ceramic dentures, fully ceramic dental items such as full crowns, part crowns, temporary fillings and lining shells may be prepared in sinter processes, using the same ceramic materials on refractory ceramic molding materials.
Dental ceramics of this type, for lining, generally require high coefficients of thermal expansion, which should be 13 to 14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C., due to the material in the metallic framework. Normally, these ceramics are processed at 950.degree..+-.30.degree. C., but this is too high for some dental alloys. It is very difficult to lower the processing temperature significantly by altering the composition of the dental ceramics without adversely altering the coefficients of thermal expansion and resistance to corrosion. Furthermore, biologically suspect components, such as e.g. lead oxide, must be avoided.
Coefficients of thermal expansion of 13-14.multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C. are generally only achieved if the dental ceramic materials contain 25 to 30% by weight of alkali metal oxides. However, in an acid environment, such as e.g. in oral cavities under plaque, this means that the limit of resistance to corrosion of these ceramics is reached.
DE-OS 39 11 460 discloses dental ceramic materials for the preparation, correction and repair of metal ceramic and fully ceramic dentures, which contain, apart from SiO.sub.2 as the balance, 5-15% Al.sub.2 O.sub.3, 0.5-2.5% B.sub.2 O.sub.3, 0.5-2.5% Sb.sub.2 O.sub.3, 0.1-0.5% CaO, 0.5-2.5% BaO, 5-10% Na.sub.2 O, 10-15% K.sub.2 O, 0.1-0.5% Li.sub.2 O and 0.1-0.5% F.sub.2. These actually have coefficients of thermal expansion of 13.5 (.+-.1).multidot.10.sup.-6 .multidot.K.sup.-1 between 25.degree. and 500.degree. C. but their processing temperatures of 730.degree..+-.30.degree. C. are still too high for some dental alloys during lining.
In German Patent Application P 40 31 168.6, which has not been published previously, ceramic materials for lining metallic dentures, which have a coefficient of thermal expansion of 16-17.5.multidot.10.sup.-6 .multidot.K.sup.-1 and a processing temperature of 770.degree..+-.70.degree. C. are described They consist of 60 to 68% SiO.sub.2, 10 to 15% Al.sub.2 O.sub.3, 0.7 to 1.5% B.sub.2 O.sub.3, 0 to 0.5% Sb.sub.2 O.sub.3, 0 to 0.5% CeO.sub.2, 0 to 0.5% BaO, 0.1 to 0.5% CaO, 9 to 12% K.sub.2 O, 9 to 11% Na.sub. O, 0.8 to 1.4% Li.sub.2 O and 0.2 to 0.4% F.sub.2.